Since diets high in protein are often prescribed for patients with osteoporosis and can cause hypercalciuria in normal subjects, a study was undertaken to investigate the alteration in calcium balance induced by dietary protein in patients with osteoporosis and in normal volunteers. With the high protein diet, the mean urinary calcium of normals increased from 144 to 259 mg per day; that of the osteoporotics from 93 to 187 mg per day. Calcium absorption did not change with increased dietary protein in the normals; it rose by 12 per cent with the high protein diet in the osteoporotics. Mean calcium balance was more negative with the high protein diet in both groups. It is concluded that excessive dietary protein has a marked effect on urinary calcium excretion and may be a factor in promoting negative calcium balance in osteoporosis.